Different base stations of a cellular telecommunication network need to have different physical Cell IDs assigned in downlink in order to avoid scrambling code collisions. A scrambling code collision would occur if two neighboring cells use the same scrambling code, which typically has a one-to-one relation to the Cell ID. In case of a scrambling code collision the signals from different base stations cannot be distinguished by a user equipment (UE) or at least the signals originating from different base stations cause a strong interference that is not averaged out due to different scrambling codes. In the worst case a UE detects the signal from the other base station rather than the signal from the own serving base station.
The problem of scrambling code collisions can also occur in connection with home base stations. In connection with Long Term Evolution (LTE) telecommunication networks a home base station is often called a Home evolved NodeB (HeNB). In connection with Universal Mobile Telecommunication Network (UMTS) a home base station is often called a Home NodeB (HNB). Such a HeNB can span a typically small home cell within the overlaying Wide Area (WA) telecommunication network. A small home cell is often referred to as a femto cell.
When setting up a new base station within a cellular telecommunication network a proper network planning has to be accomplished in order to avoid an assignment of the same Physical (cell) ID respectively the same scrambling code in particular to neighboring base stations or to base stations, which are situated comparatively close to each other.
The simplest method for assigning Physical Cell ID's consists of randomly selecting the physical ID of the base stations among available values. In case of a LTE telecommunication network 504 different values are provided by the current LTE standard specification. However, such a random selection method may lead to serious problems caused by scrambling code collisions in particular if the number of deployed base stations is high.
The publication “Solution(s) to the 36.902's Automated Configuration of Physical Cell Identity Use Case”, R3-080812, presented at the 3GPP TSG-RAN WG3 Meeting #59-bis in Shenzen, China, 31 Mar.-3 Apr. 2008 discloses different solutions for the physical cell ID selection procedure for wide area networks. Therein, also so called Self Organizing Networks (SON) are described, wherein during deployment of a plurality of base stations proper physical ID's are automatically assigned to the base station in order to avoid scrambling code collisions.
In connection with home base stations, which are deployed or which are simply only activated within the spatial coverage of a WA cellular telecommunication network, one straightforward approach for allocating proper physical cell ID's is that UEs connected to the home base station report to the home base station which codes are used for neighbor home base stations and probably also for WA base stations. However, then the home base station is assumed to be already at least partially in operation. This implies that the home base station already has selected at least an intermediary scrambling code. In case of collisions after receiving UE measurement reports of course a reconfiguration of the corresponding base station would be needed.
Another approach for allocating proper physical cell ID's is based on the fact that many home base stations have an integrated UE like receiver. During an initial setup phase of the home base station and before fully activating the home base station, the UE like receiver scans the radio environment for neighboring home base stations and WA base stations. Based on the primary and secondary synchronization signals of these base stations an appropriate physical ID respectively an appropriate scrambling code for the respective home base station can be evaluated. However, this approach has the disadvantage that a UE connected to the home base station, which has been activated or which is supposed to be activated, might receive different neighboring home base stations or even different WA base stations depending on the position of the UE in the spatial range of the home base station.
The allocation of an appropriate physical ID respectively an appropriate scrambling code to an activated home base station or WA base station can also be carried out based on synchronization information, which is exchanged by means of suitable synchronization signals between different base stations. Therefore, when improving the transmission and/or the reception of synchronization signals the allocation of appropriate physical ID's respectively scrambling codes could also be improved.
There may be a need for improving the transmission of synchronization signals within a cellular telecommunication network.